Alkyl mercaptan and dialkyl monosulfides are useful as intermediates in the production of various end products. For example, methionine, an important component in poultry feed, can be prepared in a relatively cost-efficient manner using methyl mercaptan prepared pursuant to this invention.
The reaction of alkanols and hydrogen sulfide in the presence of catalytic materials to produce alkyl mercaptan and dialkyl monosulfides has been known for almost ninety years. Modern commercial methods for the production of alkyl mercaptan and dialkyl monosulfides from alkanols and hydrogen sulfide generally employ alumina-based catalysts. As the reaction rate in the presence of these modem catalysts is quite high and the reactions themselves are extremely exothermic, heat is generated at a high rate. In addition, the reaction rate rises with temperature, thus in the absence of efficient cooling, a small temperature rise may be quickly amplified. It is therefore desirable to employ a cooling method which has not only a high heat removal capacity, but also the ability to quickly stabilize the reaction temperature in the event of even a small temperature rise.
Some present commercial methods of producing methyl mercaptan and dimethyl sulfide utilize an evaporative cooling method by supplying methanol as a liquid/vapor mixture. However, in practice, fine control of reaction temperature is limited by the difficulty in providing precise mixtures and altering said mixtures quickly and accurately. Furthermore, such methods strongly recommend the partitioning of the reaction zone into smaller regions in order to obtain sufficient cooling efficiency.
Other cooling methods utilized in modern commercial processes involve a non-adiabatic transfer of heat across an interface to an external coolant. The hot reaction products transfer heat to the cooling medium only indirectly through an intermediate separator. Reactors so cooled are susceptible to spatially uneven cooling, leading to "hot spots" which can dramatically reduce the life of the catalyst. In addition, the inability of coolant and reactant molecules to mix, as well as the indirectness of reactant-coolant heat transfer force a thermodynamic limitation on the rate and energy-efficiency of cooling attainable with such methods. As a result, the ability to respond quickly and efficiently to small temperature changes is also limited. Such non-adiabatic reactor designs are also significantly more expensive than simpler adiabatic designs.
Furthermore, sources of hydrogen sulfide tapped for industrial use can contain very high amounts of contaminants. In particular, carbon dioxide can be present in amounts of 50% by mole or higher. Carbon dioxide and hydrogen sulfide react efficiently in the presence of aluminum oxide catalysts to form another contaminant, carbonyl sulfide (J. Lavalley et al. J. C. S. Chem. Comm. 1979, pgs. 146-148)--at high carbon dioxide to hydrogen sulfide ratios, the formation of carbonyl sulfide is deemed very likely (M. Suckow et al. Separation Technology 1994, pgs. 143-151). In order to produce a product of acceptable purity, it would seem advisable to remove the carbon dioxide contaminant before the reaction process is carried out. However, not only does separation of carbon dioxide from reactants of similar weights and vapor pressures require time and energy, but once removed, the resultant carbon dioxide is a resource which may be wasted unless the costly steps of storing it and/or transporting it to another utility are undertaken.
It would thus be a significant advance in the state of the art if a process for producing alkyl mercaptan and/or dialkyl monosulfides could be found which has among its benefits increased cooling efficiency, improved capacity to respond quickly and efficiently to temperature changes, simple reactor design and beneficial utilization of the carbon dioxide contaminant, while maintaining a high rate and quality of product output.